Add stopAtFileSystemBoundary

I would like the plugin to search parent directory (if everything else fails) and search for .git file/folder or until root folder is found. I've added stopAtFileSystemBoundary property to turn on this behaviour.
Also I found a bug in resolving submodule - relative path should be constructed using .git file's parent folder.
I added more logging.
Submodules are resolved also when .git was not specified manually.
Manually configured directory should not trigger searching even if it does not exists - makes debugging hard.
No new automated tests though :(